In digital microwave beam transmissions, the bit error rate is measured in order to estimate the quality of a link. If the quality is bad, the information is sent over a backup channel. It then becomes important not to degrade the content of the message when switching from the normal channel to the backup channel.
Since the quality of a link deteriorates relatively slowly, advantage is taken of the breathing space to attempt to put the pulse train of the backup channel into synchronization with the train of the channel which is deteriorating. Once this has been done, a switchover is performed without the user being aware of it.
The invention seeks to increase the range over which two such binary trains can be synchronized.